The present invention relates to phased array ultrasonic inspection, and more particularly to a novel two dimensional phased array transducer and inspection method for the non-destructive evaluation of the volume of a material or test piece using focused ultrasonic beams.
When inspecting and/or testing materials which produce a degree of back scattering, such as, for example, titanium, steel or nickel-base super alloys, focusing of the ultrasonic inspection beam enhances the back-reflected signal from flaws contained in the test material and also reduces the noise produced by the test material. This improves the signal to noise ratio (SNR) for all ultrasonic indications, and in turn improves the capability of detecting flaws and the probability of detection (POD), allowing for the detection of flaws having reflectivities equivalent to #1 ( 1/64″) or #2 ( 2/64″) flat bottom holes. Such inspections are often required for aircraft materials, particularly those found in the rotating components of a jet engine.
However, in addition to these benefits, the reduction in insonified material volume also reduces the volume of test material that is interrogated by each ultrasonic pulse. Therefore, many focused interrogating pulses are required to inspect the volume of material under test. The conventional ultrasonic solution involves a mechanical scanning system using a series of single elements, spherically focused probes, each focused at a different depth in the material and spaced such that each individual probe setup produces a focal zone that slightly overlaps the next to produce a uniform insonification over the entire depth of interest. Each of these individual zone setups is then scanned over the surface of the material to finally produce a three dimensional volume of data for the test object. This technique is generally referred to as a “multizone” inspection and is described in U.S. Pat. No. 5,533,401A.
The primary drawback to such a technique is the time involved in setting up and scanning the entire set of zones for the given volume of material under test. An additional drawback is the difficulty in accommodating for surface curvature or other complex geometry on the surface of the test piece. This will typically require the use of either custom transducers, or complex mirror arrangements to produce the desired focal characteristics in the material. Thus, it is desirable to increase the speed at which focused beams can scan in order to reduce the inspection times, while retaining the improvements in POD for critical flaws, and accommodating for surface curvature and complex geometry.
Phased arrays are recognized as having the ability to scan and manipulate focal properties in an electronic fashion and on a shot by shot basis, thus replacing the mechanical motion of a scanner and the fixed focus of a conventional transducer. Thus, it is conceivable to replace the multiple mechanical setups and multiple transducers of the multi-zone technique with a single phased array transducer, and achieve the ultimate goals of a reduced inspection time, and easy accommodation of surface geometry.
The ability of a phased array to produce a focused beam at a significant distance from that array is primarily dependent on its element configuration and the spatial extent of the array aperture with respect to the radiated wavelength. Since focusing cannot be accomplished in an aperture's far-field it is necessary to produce arrays that can place the inspection volumes well within an aperture's near-field if that inspection volume is to be interrogated by focused beams. Furthermore, in order to generate and manipulate focal geometries in a three dimensional sense within the volume of material it is necessary that the array elements be positioned in at least a two dimensional pattern. Large area two-dimensional arrays require large area elements and/or transducers to make up the array, since electronic design limitations tend to fix the number of elements and/or transducers that can be addressed and phased. These relatively large elements and/or transducers will in turn have directivity functions of their own which limit their ability to focus over a range of angles and therefore depths.
Accordingly, the need exists for unique array architectures and focusing lenses to achieve significantly large transducer apertures within the available state of the art beamforming capabilities.